A plasma torch which has previously been used in a plasma cutting machine is so constructed as shown in FIG. 1 of the drawings attached hereto, and is provided in its central portion with an electrode 1, inside of which there is formed a cooling chamber 8. Also, outside of the electrode 1 there is formed a plasma gas passage 2, and a nozzle 3 is disposed so as to surround the electrode 1 via the plasma gas passage 2. Also, outside of a forward end of the nozzle 3 there are formed a cooling chamber 9 and a secondary gas passage 4 along with a shield cap 5 surrounding the cooling chamber 9 and the secondary gas passage 4.
A cutting process with a plasma torch of such a construction is carried out by generating a plasma arc 7 that constitutes a main arc between the electrode 1 and a workpiece 6 while causing a plasma gas 20 to flow through the plasma gas passage 2. The plasma arc 7 is pinched and thereby narrowed and densified with an orifice 3a of the nozzle 3 and is elevated in temperature and accelerated therethrough so as to be flushed towards the workpiece 6 and so as to melt and remove a portion thereof for cutting it.
Then, a water coolant is circulated through the cooling chambers 8 and 9 which are provided in the interior of the electrode 1 and the exterior of the nozzle 3, respectively, so that they may both be cooled. Also, a secondary gas 21 is then flushed through the secondary gas passage 4 provided inside of the shield cap 5 so that the above mentioned plasma arc 7 may be surrounded by the secondary gas 21.
The procedure of generating a plasma arc 7 as mentioned above is set forth below. First, a high frequency voltage is applied across the electrode 1 and the nozzle 3 to cause a spark discharge between them, resulting in the occurrence of a pilot arc. Floating on a flow of the plasma gas 20, the discharge spot of the pilot arc on the side of the electrode 1 is moved to the center of the forward end thereof while the discharge spot on the side of the nozzle 3 passing through the orifice 3a thereof is moved to a region of the outlet thereof, eventually reaching the surface of the workpiece 6, and thus establishing a plasma arc 7.
At the same time, the electric power between the electrode 1 and the nozzle 3 ceases being supplied. The plasma arc 7 is then pinched and thereby narrowed and densified with the orifice 3a of the nozzle 3 to result in a high temperature and high velocity flushing jet stream, which acts to form a cut groove of a small width in the workpiece 6 and to allow cutting thereof to proceed.
Then, while both the electrode 1 and the nozzle 3 are exposed to an elevated temperature by the plasma arc 7, they are, as mentioned above, cooled by the water coolant or air. Also, the electrode 1 which will be elevated by a temperature of several thousand degrees due to the thermo electron emission, in order for its wear to be lowered, is composed of a high melting point material. Such a material, if the plasma gas 20 contains oxygen, may be hafnium and, if it is a non-oxidizing gas not containing oxygen, may be tungsten.
Also, in a prior art plasma cutting process, the kind of plasma gas 20 that has been employed is related to the material of the workpiece 6. Thus, if a mild steel material is to be cut, the plasma gas 20 makes use of oxygen. If a stainless material or an aluminum material is to be cut, the plasma gas 20 makes use of a non-oxidizing gas not containing oxygen. The non-oxidizing gas may be composed of a single component gas such as argon or hydrogen or a mixture thereof.
By the way, as mentioned earlier, it should be noted that in plasma cutting, a plasma arc 7 at a high temperature and with a high velocity is flushed out of the nozzle 3, thereby locally melting a workpiece 6 and a portion of the molten metal thereof is blown off to form a cut groove therein, whereby the workpiece 6 continues to be cut.
Accordingly, it can be seen that the cutting quality of plasma cutting significantly depends on the configuration of the nozzle 3 through which the plasma arc 7 is pinched and thereby narrowed and densified for flushing out thereof. If the nozzle 3 wears so as to be deformed in configuration and the orifice 3a thereof is enlarged in diameter, the cutting quality should deteriorate.
Since the outlet of the orifice 3a of the nozzle 3 in particular largely affects the direction and the expansion of the plasma arc 7 flushed out therethrough, it should be noted that if the outlet of the orifice 3a wears even a little, the cut surface of the workpiece 6 will incline, the molten metal will become unable to be blown off completely and there will be left what is called a dross--a residue of the molten metal in a cut groove, and all of these deleteriously affects the cutting quality largely.
Also, as mentioned earlier, it should be noted that a plasma cutting machine in the prior art is designed to generate a pilot arc between the electrode 1 and the nozzle 3 before a main arc is initiated and, if an electrical conduction is established between the electrode 1 and a workpiece 6 with the pilot arc as a pilot flame, to form a plasma arc 7 constituting the main arc, and then, if this occurs, the supply of the electric power to the nozzle 3 is ceased so as to terminate the pilot arc. Thereafter, cutting will proceed with the main arc.
Therefore, with the plasma cutting machine, if a cutting operation is performed with such a main arc generated, such a pilot arc comes to be generated each time the arc is initiated.
Since the pilot arc is generated between the electrode 1 and the nozzle 3 as shown in FIG. 2 of the drawings, the spot (arcing spot) P sustaining the pilot arc 17 is exposed to the arc of a high temperature. Also, an air entraining flow 18 is produced in the vicinity of the forward end of the nozzle 3 such that air may be drawn so as to flow into the orifice 3a of the nozzle 3. For this reason, if the plasma gas is composed of a non-oxidizing gas, damage 19 may develop due to oxidation in the orifice 3a of the nozzle 3. Consequently, each time a cutting operation is carried out, the wear of the nozzle 3 unavoidably proceeds due to a pilot arc 17 which is generated when the arc is initiated.
The pilot arc 17 is generated from a spark discharge that is caused when, initially at the start of an arc, a high frequency high voltage is applied across the electrode 1 and the nozzle 3. The pilot arc 17 is generated across the shortest distance between the electrode 1 and the nozzle 3. Subsequently, floating on a flow of the plasma gas 20, the arcing spot on the side of the electrode 1 is moved to the center of the forward end thereof whereas the arcing spot P on the side of the nozzle 3 passing through the orifice 3a thereof is moved to a region of the outlet of the nozzle orifice 3a, and then stays in the vicinity of the outlet thereof until a main arc is generated.
Therefore, as shown in FIG. 2, it follows that the wear of the nozzle 3 when the pilot arc is generated is concentrated and proceeds at a portion of the outlet of the orifice 3a.
Thus, in the conventional plasma cutting machine, since a pilot arc is started each time a cutting process is carried out, the outlet portion of the orifice 3a of the nozzle 3, which largely affects the cutting quality, predominantly and continually wears off, it is unavoidable that the cutting quality will deteriorate. In order to maintain an acceptable level of cutting quality, therefore, it has been necessary to frequently exchange the nozzle 3.
Also, in cutting a mild steel material, it should be noted that the use of oxygen or a gas containing oxygen as the plasma gas 20 is customary but, as compared with a non-oxidizing gas, wear of the nozzle 3 due to a pilot arc is made further acute and requires the nozzle 3 to be replaced after only cutting operation time of several hours to several tens of hours. Thus, the need to enhance the durability of the nozzle 3 has been a big problem.
Thus, the requirement to replace the nozzle so often not only raises costs and the machine's running cost but also deteriorates the cutting efficiency arising from the time required to replace it, resulting in lowered machine productivity. Also, these are not all the deficiencies. Not only is personnel required who constantly monitors a reduction in the cutting quality due to a deterioration of the nozzle 3, but also an acute wear of the nozzle 3 constitutes a severe obstruction to the construction of an unmanned plasma cutting machine.
The present invention has been made with the foregoing problems taken into account and has as its object to provide a plasma cutting method which is capable of markedly enhancing the durability of the nozzle, maintaining an acceptable cutting quality over a prolonged time period, reducing the machine's running cost, and realizing an enhancement of the machine's productivity.